The present invention relates generally to a method and system for the real-time processing of seismic signals collected with multichannel seismic acquisition systems and to a stand-alone system for verifying the quality of such seismic signals so as to quickly and accurately determine if the acquisition parameters established for the multichannel seismic system are producing interpretable seismic data. More particularly, a method for obtaining surrogate seismic signals representative of a multichannel set of seismic signals acquired with a marine exploration system is provided.
Marine seismic exploration is a well-known method of geophysical investigation which is widely employed to locate undersea geological formations which may contain hydrocarbons. Marine seismic exploration is typically accomplished by towing a seismic source array, comprising one or more seismic wave generators such as air guns, and a sensor array having a multiplicity of hydrophones or other suitable transducers in a trailing sensor array over an area to be explored. As the seismic source array passes over the exploration area, it is periodically activated to produce seismic waves in the water which travel outward and downward through the sea floor and subterranean formations. Portions of the seismic wave energy are reflected back into the water by the sea floor and at each of the underlying subterranean formation interfaces. The returning reflected waves are detected by the hydrophones in the trailing sensor array which develop output signals from which the time of passage of the reflected wave can be determined. The propagation time of a seismic wave to and from a reflection point is directly related to the depth of that point. The sensor array can produce a plurality of such seismic signals which can be subsequently processed to generate topographical representations of the subterranean formations for analysis. The seismic source array and trailing sensor array are towed continually through the exploration area while gathering seismic data.
Marine seismic data is most typically gathered employing the common depth point technique. This is accomplished with the marine seismic exploration system previously described by periodically firing the seismic source array when it and a hydrophone of a trailing sensor array are approximately equidistant from an intermediate reflection point. Multichannel shot records of seismic signals are generated by the sensor array for each firing of the seismic source array. Each successive initiation of the seismic source array produces seismic waves which are reflected at an increasingly greater angle from the same reflection point. The multichannel shot records of seismic signals can be sorted for a particular reflection point by the common depth point technique and can be combined or stacked by subsequent data processing. A normal moveout correction function is applied to the gathered seismic signals to compensate for the slightly different path lengths of the incident reflected seismic waves producing each common depth point. Correction for normal moveout and stacking a series of seismic signals generated by the common depth point technique produces a composite seismic signal representing an ideal normal reflection of a seismic wave in which the ratios of the primary reflection return signal strength to noise and secondary return strength are improved.
Current marine seismic exploration techniques, as well as certain land seismic exploration techniques, involve the acquisition of multichannel seismic data; e.g., a shot record of 120 different seismic signals for each firing of the seismic source array. The acquisition of such multichannel seismic data is both an expensive and cumbersome process, which if done incorrectly, can be extremely costly to replicate at a later date. As such, the need has arisen to provide a real-time seismic processing system which can provide comprehensive seismic data quality control to insure that interpretable seismic signals have been acquired as well as to optimize certain of the acquistion parameters for the seismic source and seismic sensor arrays. Moreover, it would be advantageous to provide locally a preliminary interpretation of the seismic data acquired during the acquisition or to be able to economically transmit seismic data by way of satellite transmission to another locale for processing and interpretation.
In order to provide real-time processing of multichannel seismic data generally requires a computing system of considerable capacity. Moreover, satellite transmission of multichannel seismic data can be prohibitively expensive because of the vast amount of data included in multichannel seismic data. The present invention provides a solution to such obstacles by processing with a minicomputer only selected channels of the multichannel seismic data to produce surrogate seismic signals representative of the multichannel seismic signals. Since the surrogate seismic signals can represent a substantial reduction in the volume of the multichannel seismic data, satellite transmission of such surrogate seismic signals can now be done more economically.